The present invention relates to a semiconductor light emitting device and a method for producing the same, and more particularly, a semiconductor light emitting device comprising a gallium nitride type compound semiconductor for emission of blue light and a method for producing the same.
Such a gallium nitride type compound semiconductor is (1) a semiconductor comprising a compound of Ga of group III element and N of group V element, or (2) a semiconductor which comprises a GaN compound in which a part of Ga is substituted by other group III elements such as Al or In and/or a part of N is substituted by other group V elements such as P or As.
The semiconductor light emitting devices include light emitting diodes (hereinafter referred to as xe2x80x9cLEDxe2x80x9d) having pn junctions or double heterojunctions such as pin junctions, super-luminescent diodes (hereinafter referred to as xe2x80x9cSLDxe2x80x9d), and semiconductor laser diodes (hereinafter referred to as xe2x80x9cLDxe2x80x9d).
Although conventional blue LEDs are lower in luminance than red or green ones and disadvantageous for practical use, they have been improved by using gallium nitride type compound semiconductor and more specifically, doping an amount of Mg thus forming a p-type semiconductor layer with a low resistance and are now available for new applications.
A conventional gallium nitride LED has a structure shown in FIG. 7. It is fabricated by applying gaseous forms of metal organic compounds such as trimethylgallium (TMG) and ammonia (NH3) together with a carrier of H2 gas to a single-crystal substrate 51 of sapphire (Al2O3) at a low temperature of 400xc2x0 C. to 700xc2x0 C. using a metal organic chemical vapor deposition (MOCVD) method to form a low-temperature buffer layer 54 of, approximately 0.01 to 0.2 micrometer thick, comprising GaN, and applying the gaseous forms of the same materials at a high temperature of 700xc2x0 C. to 1200xc2x0 C. to form a high-temperature buffer layer 55, approximately 2 to 5 micrometers thick, comprising n-type GaN which is identical in the chemical composition to the layer 54.
A gaseous form of trimethylaluminum (TMA) is then added to the prescribed materials to deposit an n-type cladding layer 56 of, approximately 0.1 to 0.3 micrometer thick, comprising AlxGa1xe2x88x92xN (where 0 less than x less than 1) for creating a double heterojunction. Those n-type layers are prepared by depending on the fact that gallium nitride type compound semiconductor materials can be made type without addition of any n-type impurities or by simultaneous application of SiH4 gas.
Then, the same materials including a less amount of Al and a more amount of In than in the cladding layers are deposited to form an active layer 57 which is comprising, for example, GayIn1xe2x88x92yN (where 0 less than yxe2x89xa61) and lower in band gap energy than the cladding layers.
Also, a p-type impurity of Mg or Zn in the form of a metal organic compound gas of eg. bis(cyclopentadienyl)magnesium (CP2Mg) or dimethylzinc (DMZn) is added to the same gaseous materials as of the n-type cladding layers in a reaction tube to form a p-type cladding layer 58 comprising p-type AlxGa1xe2x88x92xN.
Furthermore, the same gaseous materials are applied for vapor deposition of a p-type GaN cap layer 59.
Whole surfaces of growth layers of the semiconductor material is then coated with a protective layer of eg. SiO2 and the like and annealed for approximately 20 to 60 minutes at a temperature ranging from 400xc2x0 C. to 800xc2x0 C., allowing both the p-type cap layer 59 and the p-type cladding layer 58 to be activated. After the protective layer is removed, a resist pattern is applied for assigning n-type electrodes. When the semiconductor layers are subjected to dry etching by chlorine plasma atmosphere, desired regions of the n-type GaN high-temperature buffer layer 55 are exposed as shown in FIG. 7. Finally, two electrodes 61 and 60 are formed by sputtering of a metal film such as Au or Al. The semiconductor layers are then diced to LED chips,
As understood, a conventional semiconductor light emitting device using the gallium nitride type compound semiconductor material has at back side a sapphire substrate made of an insulating material. For forming electrodes on the back side, it is hence needed to use etching or other complicated processing method
Although the sapphire substrate withstands a high temperature and is easily bonded to any type of crystal surface, the sapphire is very different from the gallium nitride semiconductor material in lattice constant, 4.758 (sapphire substrate) angstrom to 3.189 (gallium nitride type semiconductor crystal) angstrom, and also, in coefficient of thermal expansion. The difference in lattice constant may result in crystal defect or dislocation in the buffer layer stacked on the sapphire substrate as denoted by A in FIG. 8. If the crystal defect propagates to the single-crystal gallium nitride type compound semiconductor layers which are stacked on the buffer layer and act as operating layers, operating region is declined and also optical characteristics of the semiconductor layers degrade
In addition, the sapphire substrate is hardly cleft and it is thus not easy to produce semiconductor light emitting device chips by cleaving above-mentioned structure of the semiconductor layers. It is said that the conventional semiconductor layer structure described above is not appropriated for producing particular devices such as semiconductor laser devices in which two opposite sides are required to be mirror surfaces which are parallel with each other at high accuracy. It is also hard to process the sapphire substrate which may thus be processed with much difficulty.
It is an object of the present invention to provide an improved semiconductor light emitting device and a method for producing the same wherein the above disadvantages are eliminated and the generation of undesirable artifacts including the crystal defect and dislocation which may result from mismatch in lattice constant or thermal expansion coefficient are minimized.
It is a further object of the present invention to provide a semiconductor light emitting device and a method for producing the same, the semiconductor light emitting device having multilayer structure wherein processing like as separating wafers to chips easily by cleaving, for example, is easily performed. Consequently, gallium nitride type compound semiconductor according to the present invention enables to obtain mirror surfaces as end surfaces by cleaving for a semiconductor light emitting device which needs, like semiconductor laser, two mirror surfaces which are parallel with each other as end surfaces of the device.
A semiconductor light emitting device according to the first aspect of the present invention in order to achieve the object comprises a single-crystal silicon substrate, an insulating layer formed on the single-crystal silicon substrate, and gallium nitride type compound semiconductor layers provided on the insulating layer.
It is preferable to employ a single-crystal silicon substrate of which (111) crystal plane is a principal plane, since an insulating layer of which lattice matching with gallium nitride type compound semiconductor layer at an interface between the gallium nitride type compound semiconductor substrate and the insulating layer is appropriate can be obtained.
The gallium nitride type compound semiconductor layers may be a plurality layers including a p-type layer and an n-type layer and an active layer for emission of light This structure is preferable so as to provide the light emitting device The gallium nitride type compound semiconductor layers comprises a buffer layer, a lower cladding layer, an active layer, an upper cladding layer, and a cap layer.
The buffer layers are made of n-type GaN, the lower cladding layer is made of n-type AlxGa1xe2x88x92xN (0 less than x less than 1), the active layer is made of GanIn1xe2x88x92nN (0 less than nxe2x89xa61), the upper cladding layer is made of p-type AlxGa1xe2x88x92xN (0 less than x less than 1), and the cap layer is made of p-type GaN, thus the light emitting device with double hetero structure can be provided.
A method for producing the semiconductor light emitting device according to the first aspect of the present invention comprises the steps of:
(a) forming an insulating layer on a single-crystal silicon substrate;
(b) forming a gallium nitride type compound semiconductor layer as a buffer layer on the insulating layer;
(c) stacking on the buffer layer in sequence a lower cladding layer, an active layer, an upper cladding layer, and a cap layer, these layers being made of gallium the nitride type compound semiconductor;
(d) exposing a predetermined surface of the buffer layer by etching perpendicularly to the single-crystal silicon substrate;
(e) forming electrodes on both the cap layer and the predetermined surface of the buffer layer exposed by the etching treatment in step (d), whereby obtaining a semiconductor wafer having multilayer structure; and
(f) separating the semiconductor wafer to chips by dicing or by cleaving.
According to the first aspect of the present invention, the insulating layer of, for example, silicon nitride or aluminum oxide is deposited on the single-crystal silicon substrate and then, single-crystal gallium nitride type compound semiconductor layers which are operating layers are grown on the gallium nitride type compound semiconductor buffer layers which are on the insulating layer. As a result, the deposited layers on the substrate are similar to one another in lattice constant and coefficient of thermal expansion, thus have less chance of undesirable lattice defect and dislocation.
The buffer layer is provided for preventing crystal defect generated by lattice mismatch between the gallium nitride type compound semiconductor layers and the insulating layer on the single-crystal semiconductor substrate from extending across the single-crystal gallium nitride type compound semiconductor layers which are operating layers and from generating other defect or dislocation. The buffer layer may have a double layer structure including the low-temperature buffer layer and the high-temperature buffer layer to minimize and relax the lattice mismatch efficiently.
Also, since the single-crystal semiconductor layers of the buffer layer and the cladding layer, thickness of each of the buffer layer and the cladding layer being at least one micrometer, are identical to each other in chemical composition, their cleft edges are highly planar in cleft surface, thereby mirror surfaces being easily obtained.
To achieve the foregoing object, according to the second aspect of the present invention, a gallium nitride type compound semiconductor substrate is employed as the semiconductor substrate and gallium nitride type compound semiconductor layers are stacked on the substrate.
The semiconductor light emitting device according to the second aspect of the present invention comprises gallium nitride type compound semiconductor layers stacked on the gallium nitride type compound semiconductor layers.
A method for producing a semiconductor light emitting device according to the second aspect of the present invention comprises the steps of:
(g) growing a gallium nitride type compound semiconductor layer on a single-crystal semiconductor substrate;
(h) removing the single-crystal semiconductor substrate; and
(i) growing single-crystal gallium nitride type compound semiconductor layers including at least both an n-type layer and a p-type layer, on the single-crystal gallium nitride type compound semiconductor layer, with utilizing the gallium nitride type compound semiconductor layer as a new substrate.
It is preferable that the single-crystal semiconductor substrate is made of one member of selected from the group consisting of GaAs, GaP, InP and Si and has a (111) crystal plane, for optical and electrical characteristics of the gallium nitride type compound semiconductor layers which are formed thereon.
It is also preferable that the step (g) of growing the gallium nitride type compound semiconductor layer on the single-crystal semiconductor substrate may be implemented by forming the low-temperature buffer layer of the gallium nitride type compound semiconductor layer on the single-crystal semiconductor substrate at low temperature of 400xc2x0 C. to 700xc2x0 C. and forming the gallium nitride type compound semiconductor layer at higher temperature of 700xc2x0 C. to 1200xc2x0 C. so that the low-temperature buffer layer relaxes the lattice mismatch between the substrate and the low-temperature buffer layer and prevents crystal defect or dislocation.
More preferably, before the step (i) of growing the gallium nitride type compound single-crystal semiconductor layers, the low-temperature buffer layer of the gallium nitride type compound semiconductor is formed at low temperature of 400xc2x0 C. to 700xc2x0 C. and then, the high-temperature buffer layer of the gallium nitride type compound semiconductor is formed at high temperature of 700xc2x0 C. to 1200xc2x0 C. and is followed by the growing of the single-crystal semiconductor layers of gallium nitride type compound to minimize crystal defect or dislocation produced in the gallium nitride type compound semiconductor substrate
The single-crystal gallium nitride type compound semiconductor layers including at least both the n-type layer and the p-type layer comprise the n-type cladding layer, the active layer, and the p-type cladding layer, these three layers forming a sandwich structure. In particular, the band gap energy of the active layer is smaller than that of the n-type cladding layer or the p-type cladding layer. Also, the n-type cladding layer, the p-type cladding layer, and high-temperature buffer layer and the gallium type nitride type compound semiconductor substrate are the same in chemical composition, thus providing light emitting device with high efficiency of light emission
It is preferable that a semiconductor wafer on which the single-crystal gallium nitride type compound semiconductor layers is then cleft to desired chips, thus providing the mirror end surfaces.
According to the second aspect of the present invention, after the gallium nitride type compound semiconductor layer is grown on a single-crystal semiconductor substrate, the single-crystal semiconductor substrate is removed and single-crystal gallium nitride type compound semiconductor layers which are operating layers are provided on the gallium nitride type compound semiconductor layer which is now utilized as a new substrate. The semiconductor layers are similar to one another in lattice constant and coefficient of thermal expansion, thus having less chance of undesirable lattice defect and dislocation.
A crystal defect in the gallium nitride type compound semiconductor layer which is grown on the original single-crystal semiconductor substrate and is utilized as a new substrate may be generated by lattice mismatch between the gallium nitride type compound semiconductor layer which is the new substrate and the single-crystal semiconductor substrate. The crystal defect also may extend across the single-crystal gallium nitride type compound semiconductor layers which are operating layers. Thereby other dislocation or crystal defect may be generated. The crystal defect or dislocation is however prevented by the presence of the low-temperature buffer layer and high-temperature buffer layer provided between the gallium nitride type compound semiconductor layers and the new substrate.
Since the single-crystal semiconductor layers of the buffer layer and the cladding layer, thickness of each of the buffer layer and the cladding layer being at least one micrometer, are identical to each other in chemical composition, their cleft edges are highly planar in cleft mirror surface.
To achieve the foregoing object, the third aspect of the present invention by use of a group II-VI compound semiconductor substrate as a semiconductor substrate.
The semiconductor light emitting device according to the third aspect of the present invention comprises gallium nitride type compound semiconductor layers stacked on a II-VI compound semiconductor substrate.
The gallium nitride type compound semiconductor layers may be stacked on a top surface of the substrate comprising group VI atoms of the group II-VI compound material so that the lattice matching at interface is desirable.
The group II-VI compound semiconductor substrate nay be made of ZnSe. When the group II-VI compound semiconductor substrate is made of ZnSe, the substrate does not absorb light with at least 470 nanometer wavelength. When the semiconductor substrate is made of ZnS, the substrate does not absorb light with at least 320 nanometer wavelength. As a result, the light emitting semiconductor device can be provided which has improved efficiency of light emission
A method for producing the semiconductor light emitting device according to the third aspect of the present invention comprises the steps of:
(j) preparing the group II-VI compound semiconductor substrate;
(k) stacking a buffer layer of gallium nitride type compound semiconductor on a principal plane of the group II-VI compound semiconductor substrate;
(l) stacking on the buffer layer in sequence a lower cladding layer, an active layer, an upper cladding layer, and a cap layer, these layers being made of the gallium nitride type compound semiconductor with matching crystal lattice of each layer to one another;
(m) forming electrodes on both the top of the cap layer and the bottom of the group II-VI compound semiconductor substrate, whereby obtaining a semiconductor wafer having multilayer structure; and
(n) cleaving the semiconductor wafer to chips.
Preferably the step of forming the buffer layers comprises steps of forming a low-temperature buffer layer at low temperature and forming a high-temperature buffer layer at high temperature.
According to the third aspect of the present invention, the substrate on which gallium nitride type compound semiconductor layers are grown is made of a group II-VI compound semiconductor such as ZnSe or ZnS. The gallium nitride type compound semiconductor layers are hence similar to the group II-VI compound semiconductor substrate in both lattice constant and coefficient of thermal expansion, having less chance of undesirable crystal defect and dislocation.
Also, the group II-VI compound semiconductor substrate is employed with a top surface comprising the group VI atoms of the group II-VI compound semiconductor substrate, thus decreasing the lattice mismatch to the gallium nitride type compound semiconductor layers. This allows any crystal defect or dislocation to rarely occur into the single-crystal gallium nitride type compound semiconductor layers which are operating layers.
Since the single-crystal semiconductor layers of the buffer layer and the cladding layer, thickness of each of the buffer layer and the cladding layer being at least one micrometer, are identical to each other in chemical composition, their cleft edges are highly planar in cleft surface, thereby mirror surfaces being easily obtained
According to the fourth aspect of the present invention so as to achieve the prescribed object, a group III-V compound semiconductor substrate is used as a semiconductor substrate.
The semiconductor light emitting device according to the fourth aspect of the present invention comprises gallium nitride type compound semiconductor layers stacked on a group III-V compound semiconductor substrate.
It is preferable that the gallium nitride type compound semiconductor layers are stacked on a top surface of the substrate comprising group V atoms of the group III-V compound material, thus providing a desirable lattice matching at interface.
The group III-V compound semiconductor substrate is preferably made of a member selected from the group consisting of GaAs, InAs, GaP and InP.
A method for producing the semiconductor light emitting device according to the fourth aspect of the present invention comprises the steps of:
(o) preparing the group III-V compound semiconductor substrate;
(p) stacking a buffer layer of gallium nitride type compound semiconductor on a principal plane of the group III-V compound semiconductor substrate;
(q) stacking on the buffer layer in sequence a lower cladding layer, an active layer, an upper cladding layer, and a cap layer, these layers being made of gallium nitride type compound semiconductor substrate, with matching crystal lattice of each layer to one another;
(r) forming electrodes on both the top of the cap layer and the bottom of the group III-V compound semiconductor substrate, whereby obtaining a semiconductor wafer having multilayer structure;
(s) cleaving the semiconductor wafer to chips.
According to the fourth aspect of the present invention, the substrate on which gallium nitride type compound semiconductor layers are stacked is made of a group III-V compound material such as GaAs, InAs, GaP, or InP. The gallium nitride type compound semiconductor layers are hence similar to the group III-V compound semiconductor substrate in lattice constant and coefficient of thermal expansion, having less chance of undesirable crystal defect and dislocation. In particular, the thermal expansion coefficient of above-mentioned group III-V compound material is approximate to that of GaN as compared with conventional sapphire substrate, and distortion in the lattice will be minimized during the heating process in fabrication process.
Also, the substrate is provided with a plane corresponding to C plane of the sapphire and a top surface comprises the group V atoms of the group III-V compound semiconductor substrate, thus decreasing the lattice mismatch to the gallium nitride type compound semiconductor layers. This allows any crystal defect or dislocation to rarely occur in the single-crystal gallium nitride type compound semiconductor layers which are operating layers. The group V atoms of the group III-V compound semiconductor substrate on the top surface of the substrate promote nitrogenizing on the substrate thus producing optimum alignment of crystalline planes in the gallium nitride type compound nonconductor layers.
Since the single-crystal semiconductor layers of the buffer layer and the cladding layer, thickness of each of the buffer layer and the cladding layer being at least one micrometer, are identical to each other in the chemical composition, their cleft edges are highly planar in cleft surface, thereby mirror surfaces being easily obtained.
In addition, when the substrate is made of a proper material comprising group III atoms which are founded in the buffer layer, distortion between the substrate and the buffer layer can be minimized.